Voltage converters are frequently used to convert higher supply voltages to lower operating voltages. In particular, voltage converters are used for the control of light-emitting diodes, LED. In order to attain a highest possible energy efficiency, it is necessary to operate an LED with an operating voltage in accordance with its characteristic curve. For LEDs typically used, in particular, in mobile radio devices, the operating voltages differ in accordance with the color of the LED. Thus, for example, the voltages which are needed for green or blue LEDs are similar, in the range of approximately 3.1 V, whereas the operating voltage of a red LED, with approximately 1.8 V, is clearly below.
In known circuits, therefore, green and blue LEDs are connected together to a capacitive charge pump or to an inductive voltage converter in order to increase the available battery voltage of the mobile radio device, in case the battery becomes weak. If the battery voltage is still available at the full level or with more than the needed operating voltage of the diodes, then, the excess energy is burned, that is, converted into heat, in a current source connected with a light-emitting diode. This negatively affects the energy efficiency of the solution. In order to also control a red LED efficiently, a separate inductive step-down converter would have to be provided. This increases the costs of the overall solution.